1. Field of the invention
The invention concerns a data transmission method employing a synchronous frame combining asynchronous cells with isochronous channels on a time-division multiplex interface for which the basic frame represents a primary bit rate of 2.048 Mbit/s.
2. Description of the related art
CCITT Recommendation G.704 defines various synchronous frame structures for transferring digital signals in the form of octets.
The frame defined by Recommendation G.704 for 2.084 Mbit/s bit rate interfaces provides 32 isochronous channels each associated with a numbered time slot in each of the successive frames, the period of the frames being 125 .mu.s.
The consecutive frames are grouped into multiframes each comprising sixteen frames and some channels, including the first (No. 0) channel and in some cases the fifteenth (No. 16) channel can be divided into lower rate channels, used in particular to transmit frame and multiframe alignment bits or signalling if necessary.
Isochronous channels (e.g., channels separated by fixed intervals) are reserved during frames if at least some of the octets to be transmitted represent signals transmitted from primary PCM multiplex equipment as defined by CCITT Recommendation G.732, enabling the transmission of sampled and encoded voice signals in the form of octets. A 64 kbit/s channel can then transmit encoded signals relating to one direction of a call set up between two telephone sets, for example.
Channels are also reserved during frames if at least some of the octets to be transmitted are originated by equipment for multiplexing digital data formed into packets and specifically into cells according to the IEEE standard 802.6.
These cells are designed for asynchronous transmission of data packets with a fixed length of 53 octets, divided between five service octets at the start of the cell and 48 octets for the data to be transmitted.
Under some operating conditions it may be advantageous to transmit simultaneously synchronous channel and isochronous cell octets, for example to avoid the use of two links respectively assigned to the channels and to the cells, provided that the respective bit rates authorize simultaneous transmission on a single link, and subject to the availability of an appropriate organization.